1. Field of the Invention
The present invention relates generally to medical methods. More particularly, the present invention relates to improved methods for treating normal pressure hydrocephalus (NPH) by removing cerebrospinal fluid (CSF) from a CSF space of a patient while pressure within the CSF space remains within a normal range.
NPH is a well recognized but relatively rare condition involving an enlargement of the ventricles with little or no increase on pressure in the ventricles. It is primarily a condition of the aged although it can but rarely occurs in the young. It is characterized by a “triad” of symptoms including apraxia of gait, urinary incontinence, and dementia. Although this triad of symptoms characterizes this disorder, it is not necessary for all three symptoms to be present to make the diagnosis. In contrast to hydrocephalus in infants and an adult-onset form which are characterized by ventricular enlargement with excessive CSF volume and elevated intracranial pressures, NPH is commonly thought to be associated with “normal” CSF pressures. Such conventional wisdom, however, is misleading since NPH is typically associated with normal CSF pressures during the day and intermittently increased CSF pressure at night.
Despite the quite different etiologies, NPH is treated in the same manner as other forms of hydrocephalus, i.e., by the implantation of a ventricular shunt which drains CSF during periods of elevated CSF pressure. Treatment of NPH using conventional pressure-responsive shunts has proven to be only partially effective. When all three symptoms are present, one or more may not respond to treatment at all or respond to different degrees. In particular, such shunt implantation procedures will sometimes clear or reduce the symptoms of urinary incontinence and apraxia of gait, but will seldom improve or stabilize the patient's dementia. Use of these shunts also carries with it the significant and life threatening risk of over drainage complications.
For these reasons, it would be desirable to provide improved methods for treating NPH, where such methods would not only be able to improve the patient's condition relative to incontinence and apraxia of gait, but would also be more effective in treating the dementia associated with this disorder than are prior methods of treating NPH. In particular, it would be desirable to utilize improved shunts and other CSF removal devices which can remove and drain CSF under different flow conditions and patterns than those provided by prior CSF shunts utilized to treat NPH. At least some of these objectives will be met by the invention as described and claimed hereinafter.
2. Description of Background Art
Normal pressure hydrocephalus NPH is commonly treated by the implantation of traditional ventriculoperitoneal, lumboperitoneal or similar shunts which drain CSF from the ventricles or lumbar subarachnoid space at differential pressures, described later, typically at or above 35 mm H2O. Commercially available ventricular shunts which have been used for treating NPH include fixed pressure valves such as the ball-in-cone, slit, miter or diaphragm valves, fixed pressure valves that include a siphon-limiting device, gravity-compensating device or flow regulation or adjustable-pressure valves such as the Codman Hakim Programable and Medronic Strata valves. Even the adjustable pressure valves, which are adjusted non-invasively using a magnetic programmer, do not automatically adjust to the patient's changing requirements. Because of their operational pressure ranges, it would not be expected that these shunts would drain CSF during periods when the pressure in the patient's ventricle is generally normal, e.g., in a range from −170 mm H2O to 200 mm H2O. A comparison of the pressure-flow performance of a number of commercially available hydrocephalus shunt devices is presented in Czosnyka et al. (1998) Neurosurgery 42: 327–334. A shunt valve having a three-stage pressure response profile is sold under the Orbis-Sigma® tradename by Nitinol Medical Technologies, Inc., Boston, Mass. 02210 (formerly by Cordis) now manufactured and marketed by Integra Neurosciences, Inc.(Plainsboro, N.J.).
Recently, a promising treatment for Alzheimer's disease (AD) and other conditions associated with CSF toxins has been proposed. The proposed treatment relies on removal of CSF from the CSF space in order to both reduce the concentration of the suspected toxic substances and to promote turnover of the CSF and transport of other normal substances which may become toxic or deleterious under conditions of deficient CSF removal and turnover. Such CSF removal may be achieved in either of two ways. First, drainage can be provided by implanting a shunt which drains CSF at a substantially continuous flow rate so long as pressure in the CSF space remains within the normal range. Such pressure-responsive ventriculoperitoneal shunts are described in U.S. Pat. No. 6,383,159, and are available from Eunoe, Inc., Redwood City, Calif. under the COGNIshunt® tradename. Alternatively, target volumes of the CSF may be removed intermittently during predetermined time periods, as generally described in co-pending application Ser. No. 10/224,046, filed on Aug. 19, 2002, commonly assigned herewith. Use of such low and continuous flow shunts and/or such intermittent, volumetric-removal shunts for the treatment of NPH has not heretofore been proposed.
Conventional shunts used in the treatment of NPN and for high pressure forms of hydrocephalus are designed to control intracranial pressure (ICP). Such shunts must therefore be chosen, in the case non-adjustable shunts, or adjusted, in the case of adjustable shunts, to operate at a particular pressure control point (i.e., opening pressure). In this case pressure is being controlled and the flow through the shunt necessary to maintain this pressure point is variable and unknown. For this reason it is often the case that the control point of the shunt is mismatched to the patient and either too much or too little CSP is drained. These conditions are known as overdrainage and underdrainage, respectively. Overdrainage, if untreated, can have severe and life threatening consequence. Since brain anatomy may be changed, overdrainage leads to the ventricles undergoing a significant reduction in volume, becoming slit-like. They may actually close down around the holes of the ventricular catheter causing intermittent or permanent shunt obstruction. The cortical surface may move downward (due to a reduction in the size of the ventricles) which leads to a tearing of the bridging veins with concomitant development of subdural fluid collections such as subdural hematomas. Underdrainage, on the other hand leads to a lack of efficacy. The present invention postulates that NPH is not a fundamentally disease of elevated pressure, but a disease of poor CSF circulation and turnover. A shunt, as defined in the present invention, designed to provide for a known and safe low flow, would protect against both over and underdrainage while therapy to the NPH patient.